This invention relates to acoustical apparatus and methods for reproducing acoustic waves with desired patterns.
The most common methods for abating sound energy involve various passive techniques such as the mechanical process of blocking acoustic energy, or the processes of converting acoustic energy into a different form such as heat energy or to a different frequency. These approaches have limited success when the medium propagating the acoustic energy must be allowed to pass freely with only the acoustic energy being dissipated. Theoretically, more complete attenuation could be provided by an active system which supplied an additional amount of energy sufficient to cancel the acoustic energy. Attempts to provide a practical active attenuation system, by using the known principle of wave interference, have been uniformly disappointing.
Many experimentors have attempted to generate, for the purposes of wave interference, an anti wave which is 180.degree. out-of-phase with respect to an acoustic wave. While this technique will cancel the intermediate portion of a pure sine wave over an extremely limited zone in space, the theoretical and practical deficiencies of such a technique have not been recognized. A 180.degree. phase shift system will not cancel the first half cycle of an acoustic wave, nor the last half cycle of a locally generated anti wave. When the acoustic wave has a non-symmetrical pressure variation, a 180.degree. phase shift does not cancel the acoustic wave but in fact adds to the total objectionable sound energy.
Complete cancellation by wave interference, even for a limited zone in space, requires the use of an anti wave whic is in-phase and of mirror symmetry with respect to the acoustic wave to be cancelled. A few experimentors have recognized this principle, but have been unable to provide apparatus or methods capable of generating the proper antiwave. For example, it has been known to mount movable diaphragms for a microphone and for a loudspeaker in the same plane, and drive them oppositely. Such systems have not recognized the inherent time delays occurring in the energy conversion processes occurring in the microphone and the loudspeaker.
All prior attempts to provide an active cancellation system have generated an anti wave with one or more vectors of propagation at an angle to the vectors of propagation of the acoustic wave. While some cancellation may be produced where the two waves cross in space, both waves continue to propagate and at other points in space create more objectionable noise than existed in the original acoustic wave alone. The presence of walls may reflect these waves back into the limited cross-over zone, nullifying the effect of the original cancellation. Furthermore, the housings for the anti noise apparatus have themselves altered and scattered the acoustic wave propagation pattern, making any significant attenuation virtually impossible. As a result of all of these factors, prior active cancellation systems for acoustic energy have generally been of no practical or commercial use.
The suppression of sound noise created by an air breathing engine such as a gas turbine engine has been given extensive consideration. Separate noise abatement methods have evolved to solve the problems of pure jet noise, which may be characterized as originating external to the engine, and internally generated noise produced by rotating machinery such as the compressor, fan, and turbine. Noise abatement methods for internally generated noise generally concern acoustic treatment of the inlet duct, the fan outlet duct, and noise reduction at the compressor/fan source. Also utilized is the choked intake in which a stowed vane, inflatable diaphragm, or other flow restriction device is deployed so that air in an inlet reaches sonic velocity (Mach number near or greater than 1). While these devices produce some reduction over certain solid angles, the presence of movable structures in the intake is undesirable. Furthermore, this technique cannot be used to silence a fan outlet in which the noise is travelling with the air stream.
Compressor/fan noise generated by interaction of the rotor blades and stator vanes is in the form of spinning modes of one or more lobe patterns which propagate through the compressor inlet duct. The nature of compressor noise and many methods for suppressing such noise are based on the work of J. M. Tyler and T. G. Sofrin, see particularly "Axial Flow Compressor Noise Studies" appearing in SAE Transactions, 1962, pages 309-332, and U.S. Pat. No. 3,194,487 granted July 13, 1965. Such noise abatement techniques include selecting and indexing the blade and vane combinations such that spinning modes of equal intensity and 180.degree. out-of-phase will alledgedly cancel in the duct. Even when particular blade/vane combinations are physically realizable in a practical compressor, imperfect speed control and differences in radial distributions of the modes result in incomplete cancellation. Altering of the duct cut-off frequency in order to suppress discrete noise above the duct cut-off frequency has also been attempted.
While prior abatement methods have in fact reduced objectionable noise, the amount of reduction has been disappointing, especially in view of the logarithmic nature of human hearing. Equally important, desired aerodynamic properties for the engine often must be compromised in order to incorporate the noise abatement techniques. It would be desirable to aerodynamically design a compressor, fan, and turbine irrespective of noise considerations, and abate any resulting noise by techniques which do not to any significant degree restrict air flow, alter the efficiency of the engine, or require moving parts in the inlet/outlet ducts.